As Long Term Evolution Advanced, LTE-A, (Long Term Evolution advanced release) starts for commercial use, the next generation standard, 5G standard, is waiting to be issued, and various equipment manufacturers and operators are also escalating demands for mobile radio frequency, RF, system. The total amount of frequency bands of current global 2G, 3G and 4G LTE reached more than 40, and to obtain frequency band resource, it is needed to support the Carrier Aggregation, CA, and multiple antennas (multiple input multiple output, MIMO) technology and etc., and is needed for the wireless base station to have wide bandwidth, high efficiency, flexible and configurable features and etc., greatly increasing the complexity of RF front end. The full digital transmitter based on the highly efficient switch mode power amplifiers, SMPA, is the ideal solution.
Modern communication system uses a non-constant envelope modulation mode, and traditional transmitter scheme uses a power regression method to meet the requirements of its linearity, which results in the decrease of the efficiency seriously. FIG. 1 is an implement schematic diagram of all-digital transmitter of SMPA in existing art. As shown in FIG. 1, all-digital transmitter of highly efficient SMPA mainly includes: Digital Encode Modulator and SMPA. Digital Encode Modulator completes digital modulation in the digital domain, produces a modulation switch signal to drive the SMPA. Power Amplifier, PA, mainly operates in the saturation region, which has highly efficient, flexible and reconfigurable, high linear characteristics and etc.
At present, all-digital transmitter encode modulator mainly has two kinds, Delta-Sigma Modulator, DSM, and Pulse-Width Modulator, PWM.
DSM and PWM are widely used in the audio field, but it has the following problems. The main problem faced by the DSM is as follows. In the process of DSM processing, the sampling frequency demand is too high and the encoding efficiency is too low. Since the DSM uses the oversampling technology, the higher the oversampling rate is, and the smaller the out-of-band noise of the output signal is, and the better the performance of the SNR, Signal-to-Noise Ratio is. Therefore, for high bandwidth communication signals, it is required for the DSM to operate at a higher sampling frequency, and as a result, it is difficult to realize. In addition, when signal bandwidth is relatively wide, in the composition of the DSM output signal, the proportion of the useful signal energy in the whole signal energy (hereinafter referred to as the encoding efficiency) is very low, and therefore, the final system efficiency (the system efficiency is equal to the product of the signal encoding efficiency and the power amplifier efficiency) is influenced. The main problem faced by the PWM is the issue of accuracy of the digital implementation. When the applications are implemented in the field of digital communication, the signals produced by directly according to the principle and method of the analog circuit has discrete features when the signals pass a digital circuit for the clock driven, so it will produce the error and reduce the performance of signals.
In conclusion, in the field of communication, when encoding modulation is performed by using DSM or PWM, there is the problem on the performance requirements such as oversampling rate and accuracy and the like.